Power supply for an electron beam furnace gun



y 9, 1968 E. R. ANDERSON 3,392,304

POWER SUPPLY FOR AN ELECTRON BEAM FURNACE GUN Filed Oct. 19, 1965 Fig.

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ATTO RN EYS POWER The present invention relates generally to powersupplies and more particularly to a power supply which is adaptable forsupplying several kilowatts of power to an electron beam gun employed ina high vacuum electron beam furnace.

Electron beam furnaces may be utilized in numerous situationsparticularly where material treating operations are to be conducted.Typical examples of such operations which are now quite well knowninclude high temperature melting and purification of various materials,vapor plating, cleaning, etc. Usually the electron beam furnace employedin such operations comprises an electron gun disposed in an evacuatedenclosure together with the material to be treated. The electron guntypically is provided with an electron emitting cathode or filament andan accelerating anode, which is maintained at a high positive potentialwith respect to the cathode so as to establish the high electrostaticfield for accelerating the electrons. When the cathode is suflicientlyenergized and the requisite electrostatic field is established a highintensity beam of electrons is emitted from the electron gun and may bedirected to bombard the material, thereby heating the same. Obviouslythe amount of heating effected is related to the intensity of theelectron beam current together with the accelerating voltage applied bythe field through which the electrons travel.

As the electrons bombard the target material various occluded gases andvaporous materials are released from the material. The presence of suchsubstances in the path of the electrons may result in a substantialdecrease in resistance in the path of the electrons, leading to a corresponding marked increase in the electron beam current.

'This often causes arcing in the electrostatic field between the cathodeand the anode in the electron gun chamber. Such effects are generallyundesirable since they tend to shorten the life of the electron gun aswell as damaging the target material.

Apparatus has been developed for alleviating the aforementioned eifectsas well as regulating the power supplied to the electron gun. Certain ofsuch apparatus, during normal operation, maintains the voltage at agenerally constant level, and when arcing occurs limits the currentsupplied to a preselected level. Such apparatus has been relativelycomplex. I Other apparatus has been developed which controls ormaintains constant the current supplied to an electron gun. Suchapparatus normally employ monocyclic power supplies which usuallyinclude inductors and capacitors connected so as to provide resonantcircuits which resonate at the frequency of the input circuit. But,extensive means are ordinarily provided in such circuits for protectingthe various elements against overstress. In addition, reactive elementsof uneconomica'lly large size are often required.

' n Stat Patent It is an object of the present invention to provide animproved and simplified power supply adaptable for supplying severalkilowatts of power to an electron beam gun employed in a high vacuumelectron beam furnace while maintaining the electron beam currentemitted by the gun below preselected levels.

Another object of the invention is the provision of a power supply foran electron beam furnace which limits the effects of arcing in theelectron beam.

Still another object is the provision of a power supply for an electronbeam gun which limits the maximum current supplied to the gun under aredischarge conditions.

A further object is the provision of an electron beam gun power supplywhich is relatively economical to manufacture, durable in use and whichaids in prolonging the useful life of the electron gun structure.

Other objects and advantages will become readily apparent from thefollowing detailed description and accompanying drawings wherein:

FIGURE 1 is a schematic circuit diagram of one embodiment of a powersupply in accordance with the present invention, and

FIGURE 2 is a graph showing typical characteristic curves of a powerdiode suitable for use in the circuit illustrated in FIGURE 1.

Generally, in the illustrated power supply the current supplied by anelectron gun 10 in an electron furnace 12 is limited to a preselectedlevel by employing a diode 14 connected in series with the output of aD-C power supply 16 and the electron beam of the gun 10. By limiting thebeam current to a preselected level, the effects of abrupt increases inthe electron beam current, such as may occur during arcing are avoided,thus preventing damage to the electron gun structure as well as to thetarget material.

More specifically, the high voltage D-C power supply 16 may be aconventional three phase, full wave rectifier which is connected throughsuitable leads 18 to a source of three phase A-C power (not shown). Thepositive output terminal 20 of the power supply 16 is grounded and thenegative terminal 22 of the power supply 16 is coupled through aconductor 24 to the filament 26 of the diode 14.

In the illustrated embodiment, the diode has a directly heated filamentwhich is heated by a filament transformer 28. In this connection aprimary winding 30 of the filament transformer 28 is connected to afirst variable autotransformer 32, which is connected to a single phaseof the A-C power supply. A secondary winding 34 of the filamenttransformer 28 is coupled to the filament 26 of the diode 14. Thefilament transformer 28 is preferably provided with high voltageinsulation on its windings since there is generally a substantialditference between the voltage on the windings.

The anode 36 of the diode 14 is coupled through a conductor 38 to thecathode 40 of the electron gun 10 which may be of the conventional type.In the illustrated embodiment, the cathode 40 of the electron gun 10 isenergized by a filament transformer 42. The filament transformer 42includes a primary winding 44 connected to a second variableautotransformer 46 which is connected to the single phase of the ACpower supply. The filament transformer 42 further includes a secondarywinding 47 coupled to the cathode 40. The windings of the filamenttransformer 42 are also preferably provided with high voltageinsulation.

The accelerating anode 48 of the electron gun is suitably grounded sothat the requisite high voltage or accelerating potential is appliedbetween the cathode 40 and the accelerating anode 48 of the gun 10. Theelectron gun 10 is suitably disposed within the appropriately groundedelectron beam furnace 12 so that the electron beam is directed upwardlyand is deflected onto the suitably grounded target material 52 by atransverse magnetic field established by a suitable magnet 53 disposedwithin the furnace 12 together with the electron gun 10 and the targetmaterial 52. In addition, the furnace includes a suitable vacuum pump 54for maintaining the necessary vacuum.

As may be seen a complete series circuit is defined extending from thegrounded positive terminal of the power supply 16 through the negativeterminal 22, through the diode 14, and through the electron beam emittedby the cathode 40 to the grounded accelerating anode 48. Thus, the diode14 is electrically in series relationship with the electron beam andaccordingly may be appropriately adjusted to limit the electron beamcurrent, as is hereinafter explained in detail.

During normal operation of the electron gun, the beam current isadjusted by adjusting the voltage applied across the primary winding 44.This is achieved by appropriate adjustment of the second variableautotransformer 46. The maximum beam current which can flow through theelectron gun is set by selecting the level of operation of the diode.The desired maximum level of the control diode 14 is selected by meansof the first variable autotransformer 32, which controls the filamentcurrent of the diode 14. The particular power diode 14 is selected tosupply the required maximum current to the electron gun. In thisconnection, a power diode of the type having a substantially puretungsten filament is generally employed. The characteristic curves atseveral levels of filament current of a typical power diode suitable foruse in a power supply in accordance with the present invention areillustrated in FIGURE 2.

Observation of these curves reveals that the maximum diode current asmeasured along the horizontal axis, increases as the filament current(I;) increases, but reaches a maximum value for each particular value ofIf as shown by the intersection of each curve with the base line 55. Ata selected value of I for example at 0.9 I the current passing throughthe tube and correspondingly the electron beam current, may beconveniently adjusted by adjusting the second autotransformer 46 tocause the electron beam current to vary from zero amps, as shown by thepoint designated by the numeral 56, to the point designated by thenumeral 58, at which the emission current of the particular tube at thislevel of filament current is approached. Accordingly, a desired current,which is less than the maximum diode current at the selected level ofoperation, may be drawn from the anode 36 of the diode 14 by adjustingthe filament current, supplied to the cathode 40 of the electron gun.

As the diode current increases from zero amps to the amperage at thepoint 58, the voltage drop across the diode, which is a relatively lowproportion of the D-C supply voltage, increases at a relatively low rateremaining substantially constant with increasing diode current. Theremainder of the supply voltage, thus, is developed across the electrongun 10, providing a constant high accelerating potential for theelectron beam as the electron beam current is varied. The voltage acrossthe electron gun 10 is accordingly regulated or held substantiallyconstant during normal operation as the electron beam current is varied.

However, if the electron beam current abruptly begins to increase, whichmay occur as a result of an abrupt decrease in resistance between thecathode 40 and the accelerating anode 48 of the electron gun 10, causedby arcing in the electron beam, the voltage drop across the electron gun10 will rapidly decrease and approach zero. Consequently, substantiallyall of the voltage supplied by the power supply 16 would appear acrossthe diode 14. But, arcing is minimized because the current passingthrough the diode 14, which correspondingly passes through the electrongun is limited to a preselected level, as previously described.

It can be seen from the foregoing that the disclosed power supply isprovided with a protective feature which limits the maximum currentlevel which can be reached by the electron beam. This current level isgenerally dictated by the emission characteristics of a diode con nectedin series relationship with the electron gun. At the same time, thevoltage developed across the electron gun is maintained substantiallyconstant, or regulated, as the electron beam current is varied withinthe preselected limits determined by the diode characteristics.

If desired, a plurality of diodes may be connected to a common powersupply and coupled to a plurality of electron guns. This may beconveniently achieved since there is a common ground return for thetotal emission current passing through each tube to the electron gunsand thence to the grounded target material.

Thus, an improved and substantially simplified power supply has beenprovided in which, after selecting a particular maximum current level byadjusting the filament temperature of the diode, a single control isadjusted for varying the electron beam current over its preselectedoperating range. At the same time the voltage developed across theelectron gun is regulated during normal operation while the beam currentmay vary, and an automatic protective feature is provided for limitingthe maximum electron beam current to a preselected value regardless ofabrupt decreases in the resistance of the electron beam path such as mayresult from arcing.

Various changes and modifications may be made in the above-describedpower supply without deviating from the spirit or scope of the presentinvention. Various features of the present invention are set forth inthe appended claims.

What is claimed is:

1. Apparatus for controlling the electron beam current supplied by anelectron gun and directed at a target material in an electron beamfurnace comprising means for supplying a direct current voltage to saidelectron gun and a diode serially coupled between said direct currentvoltage supply means and said electron gun for lim iting the currentconducted therethrough.

2. Apparatus for controlling the electron beam current supplied by anelectron gun and directed at a target material in an electron beamfurnace comprising means for supplying a direct current voltage to saidelectron gun, a diode serially connected between said voltage supplymeans and said electron gun for limiting the current conductedtherethrough, said diode having a heated filament, and means coupled tosaid heated filament for adjusting the current supplied thereto.

3. Apparatus for controlling the electron beam current supplied by anelectron gun and directed at a target material in an electron beamfurnace, said electron gun having a cathode, said apparatus comprisingmeans for supplying a direct current voltage to said electron gun, adiode serially connected between said voltage supply means and saidelectron gun for limiting the current conducted therethrough, said diodehaving a heated filament, means coupled to said heated filament foradjusting the current supplied thereto, and means coupled to saidcathode for adjusting the current supplied thereto.

4. Apparatus for controlling the electron beam current supplied by anelectron gun and directed at a grounded target material disposed in agrounded electron beam furnace, said electron gun having a cathode andhaving a grounded accelerating anode, said apparatus comprising meansfor supplying a direct current voltage to said electron gun, said directcurrent voltage supply means having a positive output terminal connectedto ground and having a negative output terminal, a diode seriallyconnected 'between said negative output terminal of said voltage supplymeans and said electron gun for limiting the current conductedtherethrough, said diode having a filament, means coupled to saidfilament for adjusting the current supplied thereto, and means coupledto said cathode for adjusting the current supplied thereto.

References Cited UNITED STATES PATENTS 2,912,616 11/1959 Marchese et a131514 2,994,801 8/1961 Hanks 31514 3,172,007 3/1965 Hanks et a1. 315-14RODNEY D. BENNETT, Primary Examiner.

0 M. F. HUBLER, Assistant Examiner.

1. APPARATUS FOR CONTROLLING THE ELECTRON BEAM CURRENT SUPPLIED BY ANELECTRON GUN AND DIRECTED AT A TARGET MATERIAL IN AN ELECTRON BEAMFURNACE COMPRISING MEANS FOR SUPPLYING A DIRECT CURRENT VOLTAGE TO SAIDELECTRON GUN AND A DIODE SERIALLY COUPLED BETWEEN SAID DIRECT CURRENTVOLTAGE SUPPLY MEANS AND SAID ELECTRON GUN FOR LIMITING THE CURRENTCONDUCTED THERETHROUGH.